The present invention generally relates to circuit devices adapted for attachment to substrates using flip-chip techniques. More particularly, this invention relates to a flip-chip technique for attaching a circuit device having a surface region with which contact with an underfill material is to be avoided.
A flip chip is generally a monolithic surface mount (SM) semiconductor device, such as an integrated circuit (IC), having bead-like terminals formed on the same chip surface as the circuitry of the chip. The terminals, typically in the form of solder bumps, secure the chip to a substrate and electrically interconnect the flip chip circuitry to a conductor pattern formed on the substrate, which may be a ceramic substrate, printed wiring board, flexible circuit, silicon substrate, etc. The solder bumps are typically located at or near the perimeter of the flip chip on bond pads that are electrically interconnected with the flip chip circuitry. Reflow solder techniques are widely employed to initially form the solder bumps and then later form the chip-to-substrate connections after placement of the chip on the substrate. Such a technique typically entails depositing a controlled quantity of solder on the bond pads of the flip chip by electrodeposition or a thick-film deposition technique such as stencil printing, and then heating the solder above its liquidus temperature to form the solder bumps on the bond pads. After cooling to solidify the solder bumps, the chip is soldered to the substrate by registering the solder bumps with their respective conductors on the substrate, heating the solder bumps to a temperature above the liquidus temperature of the solder to cause the solder to reflow, and then cooling the solder to form solder connections that metallurgically and electrically interconnect each flip chip bond pad to a conductor on the substrate.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing and size of the bond pads, but also to control the height of the solder connections after soldering. As is well known in the art, controlling the height of solder connections after reflow is often necessary to prevent the surface tension of the molten solder from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate, often termed the “stand-off height,” is often necessary to allow the penetration of an underfill material, which fills the space between the chip and substrate to reduce thermal stresses on the solder connections. Stand-off height can be controlled by the amount of solder deposited on the flip chip to form the solder bump and/or by the use of solder stops that limit the surface area over which the solder bump is allowed to reflow.
Control of solder bump position, size, and pitch are dictated in part by the manner in which the solder is deposited on the bond pads. Fine solder bump pitches can generally be obtained with electroplating techniques that require a plating mask, typically a photoresist material. Larger solder bumps are typically formed by printing a solder paste using a stencil, typically formed of a photoimageable dry film. Both of these techniques are suitable for devices that are relatively flat and do not have a fragile surface, such as a diaphragm, beam, or other micromachine on the surface where the bumps are required. For example, if a pressure sensor is desired to be flip-chip mounted to a substrate, attempts to use a dry film process to print the solder can possibly cause yield loss due to breakage of the pressure sensor's diaphragm. While other mounting techniques are available for pressure sensors, such as wire-bonding, certain advantages associated with flip-chip technology make a flip-chip mounted sensor desirable. For example, pressure sensors that rely on capacitive sensing have an inherent high impedance and are thus very susceptible to variation in electrical charges, electromagnetic interference (EMI), and coupling at the interface between the sensor and its associated readout circuit. In addition, pressure sensors are very susceptible to stresses in the substrates on which they are mounted. A flip-chip mounted device reduces capacitive coupling effects, can achieve better stress management, and generally provides a more robust connection than wire-bonding in terms of shock and vibration resistance. Finally, the ability to flip-chip mount a pressure sensor enables the pressure sensor to be simultaneously reflow soldered with other surface mount components to a substrate, eliminating the cost and time that would be required to perform a separate wire-bonding operation.
From the above it can be seen that, while flip-chip mounting of a pressure sensor chip is desirable in terms of processing and performance, dry film techniques suitable for forming solder bumps of sufficient size are not compatible with the fragile diaphragms of the sensors. Accordingly, it would be desirable if an improved method were available for forming solder bumps on a flip-chip mounted pressure sensor, as well as other flip-chip mounted devices having fragile micromachines or otherwise sensitive surface areas.